1. Field of the Invention
The present invention relates to a metal ion plasma generator that produces a metal ion plasma by vaporizing a metal cathode by means of a vacuum-arc discharge.
2. Description of the Background Art
An apparatus shown in FIG. 8 is known which produces the metal ion plasma by the vacuum-arc discharge, in which the apparatus is composed of a metal cathode 52 of a bar-form and an anode plate 53 of a plate-form in a vacuum chamber 51 (refer to a disclosure in JP-A-63-276858). The cathode 52 has an axis perpendicular to the anode plate 53 and is supported by a cathode holder 54, The anode plate 53 has an opening 55 which is coaxial with and faces with a space to the cathode 52.
The front end part projected from the cathode holder 54 is sheathed by an insulator ring 56 made of an insulating material. And, the insulating ring 56 is further sheathed by a trigger ring 57 made of a conductive material, and a trigger electrode 58 is guided inside the vacuum chamber 51 so as to touch the outer surface of the trigger ring 57.
In the foregoing construction, applying a high pulse voltage between the trigger ring 57 and the cathode 52 through the trigger electrode 58 will generate a trigger discharge. And then, applying a specific voltage between the anode plate 53 and the cathode 52 will generate a vacuum-arc discharge between the anode plate 53 and the cathode 52, whereby the foregoing trigger discharge is served as a starter. A point called arc spot where energy is locally concentrated emerges at the front end part of the cathode 52 through this vacuum-arc discharge. This part vaporizes and ionizes to produce a metal ion plasma. This metal ion plasma is guided through the opening 55 of the anode 53 to a processing room (not illustrated) connecting with the foregoing vacuum chamber 51.
When the foregoing apparatus is used for forming a thin film, the foregoing metal ion plasma is supplied on a substrate disposed in the processing room. And, when the foregoing apparatus is used for a metal ion source, the foregoing metal ion plasma is supplied to an ion pullout electrode provided inside the processing room.
As shown in FIG. 9(a), a part of the metal ion plasma Mv flowing through the opening 55 of the anode plate 53 deposits at the edge of the opening 55, growing a metal film Mf, presenting the so-called "built-up" phenomenon as shown in FIG. 9(b). This deposition reduces the diameter of the opening 55 and the passage ratio of the metal ion, and consequently it deteriorates the characteristics of the arc source.
Further as shown in FIG. 9(c), when the forgoing metal film Mf grows thick enough to electrically short the anode plate 53 and the cathode 52, the vacuum-arc discharge stops and the operation cannot be continued. And, the short-circuit between the anode plate 53 and the cathode 52 will give a latent damage to electric components on the power supply line. Therefore conventionally, after a certain hour of continuous operation, before the short-circuit between the anode plate 53 and the cathode 52 is produced, the whole apparatus is periodically shut down to restore air pressure in the vacuum chamber 51. And, a cleaning work is done to mechanically remove a film deposited on the anode plate 53. The continuous operation hour is generally set on the basis of experiment results. The reason is that the deposition rate of the metal film Mf is different depending on the material of the metal cathode.
However, the foregoing conventional apparatus needs a frequent cleaning work to remove deposited films after restoring air pressure in the vacuum chamber 51, which reduces a continuous operation time; consequently, a sufficient productivity cannot be achieved when, for example, a thick film coating or an ion-implantation requiring a high implantation density is done. And, a maintenance work becomes complicated which takes out the anode plate 53 from the vacuum chamber 51 and removes a metal film on the periphery of the opening 55 using, for example, a grinder.